Here i´ve build a very Special O-Ring !!! 
(Cause of the old closed bolt Problem, you could have a clean kill but your first shoot is flying much shorter cause the ball sticks somewere else in the Barrel but not in the back)
It just Needs an O-Ring and a few single hairs of a brush.
For build use a Slim needle to push through the single hair and a lighter to melt the end against slide through the O-Ring.
I´ve tested dry with a reball and closed bolt and hit the Barrel tip a few times in my Hand and it still was holding the ball !!!
(Cause of the old closed bolt Problem, you could have a clean kill but your first shoot is flying much shorter cause the ball sticks somewere else in the Barrel but not in the back)
It just Needs an O-Ring and a few single hairs of a brush.
For build use a Slim needle to push through the single hair and a lighter to melt the end against slide through the O-Ring.
I´ve tested dry with a reball and closed bolt and hit the Barrel tip a few times in my Hand and it still was holding the ball !!!